In recent years, environmental problems have been highlighted on a global scale, and as one of countermeasures, reduction of power consumption or fuel consumption in industrial machines or transportation machines is taken in factories or by transportation carriers, etc. As one means to solve the above problems, there has been required a much higher effect on power saving and fuel saving by various lubricating oils used for the above machines.
Lubricating oil products have a so-called temperature dependence of viscosity that the viscosity generally greatly varies as the temperature is changed. Since the working temperature of equipment using lubricating oil greatly varies in certain cases, the temperature dependence of the viscosity is considered to be preferably small. Then, for the purpose of reducing the temperature dependence of the viscosity, a certain polymer that is soluble in a lubricating base oil has been used as a viscosity modifier for lubricating oils. In recent years, an α-olefin polymer has been widely used as such a viscosity modifier, and in order to further improve a property balance of lubricating oil, a variety of further improvements have been made (patent literature 1).
Such viscosity index improvers as above are generally used for maintaining a proper viscosity at high temperatures. On the other hand, under such circumstances that energy saving and resource saving have been strongly considered to be part of reduction of environmental burden, a viscosity modifier, which holds down viscosity increase particularly at low temperatures (is excellent in low-temperature characteristics) and is excellent also in durability, has been desired recently. In general lubricating oil applications, in order to obtain excellent low-temperature characteristics, control of a concentration of a polymer contained to the lowest is advantageous also from the economical viewpoint, and therefore, use of a polymer having a molecular weight as high as possible is known. However, an α-olefin polymer having a high molecular weight tends to be disadvantageous in terms of shear stability.
Particularly in gear oil applications among industrial lubricating oil applications, high durability (shear stability) has been required, and performance given in consideration of a balance between durability and viscosity characteristics has been desired. Further, of various lubricating oils, gear oils are used under particularly severe conditions, so that requirements for higher performance and longer life are strong, and also with regard to an extreme pressure agent that is a component exerting influence on formation of a stable oil film, further improvement in performance is desired.
As lubricating base oils, mineral oils are classified into three ranks of Groups (I) to (III), and further, poly-α-olefins (PAO) are classified as Group (IV) and the others are classified as Group (V) by the API classification. In various automotive lubricating oil applications, in order to cope with higher performance required and reduction of environmental burden, a ratio of use of Group (II) and Group (III) mineral oils or synthetic oils such as poly-α-olefins has increased though Group (I) mineral oils have been hitherto widely used. On the other hand, also in the industrial lubricating oil applications, long life and high durability are desired, and the aforesaid Group (III) mineral oils or poly-α-olefins have been used. Particularly in the recent industrial gear oils, shear stability is strongly desired as a main parameter of durability. It is difficult to meet the shear stability required herein by the use of conventional viscosity modifiers of high molecular weight type, so that α-olefin polymers of relatively low molecular weight, such as polybutene, have been used. However, there is room for improvement in viscosity characteristics of polybutene, particularly in sufficient fluidity thereof at low temperatures, depending upon the use applications.